Oscillator circuit and L load differential circuit achieving a wide oscillation frequency range and low phase noise characteristics

ABSTRACT

An oscillator circuit is formed of a differential LC resonant circuit formed of an L load differential circuit including inductance-variable portions and a capacitor element, and a positive feedback circuit formed of N-channel MOS transistors. The inductance-variable portion is configured to vary the inductance by selecting a plurality of switch circuits arranged between a plurality of arbitrary positions on a spiral interconnection layer and the input/output terminal, and thereby can control an oscillation frequency. The inductance-variable portions form an inductor pair when the switch circuit among the switch circuits coupled between the first input/output terminals is turned on together with the switch circuit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of Application Ser. No.11/280,410, filed Nov. 17, 2005, which in turn is a DivisionalApplication of Application No. 10/644,865, filed Aug. 21, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an oscillator circuit and an L loaddifferential circuit, and particularly to an oscillator circuit using anLC resonant circuit as well as an L load differential circuit mountableon the oscillator circuit.

2. Description of the Background Art

In wireless devices such as a cellular phone, a local oscillator circuitis used for frequency conversion of received signals into low-frequencysignals allowing demodulation and for frequency conversion of sendsignals (i.e., signals to be sent) into high-frequency signals, and isrequired to have a wide oscillation frequency range and can lower noises(phase noises) at and around an oscillation frequency.

A Voltage Control Oscillator (VCO), which is a kind of local oscillatorcircuit, utilizes an oscillation phenomenon caused by positive feedbackof the circuit, and can control the oscillation frequency by a controlsignal. In general, the VCO employs a resonant circuit or utilizes adelay time of a circuit.

In connection with the VCO utilizing the resonant circuit, a negativeconductance LC oscillator circuit is known as an oscillator circuitutilizing negative resistance characteristics of a positive feedbackcircuit formed of transistors, as disclosed, e.g., in A. Yamagishi etal., “A Low-Voltage 6-GHz-Band CMOS Monolithic LC-Tank VCO Using aTuning-Range Switching Technique”, IEICE Trans. Fundamentals, vol.E84-A, No. 2, February 2001. Since this oscillator circuit uses the LCresonant circuit including an inductor element and a capacitor element,it can achieve good phase noise characteristics, and application to VCOsfor portable cordless devices has been expected.

A structure and an operation of a conventional VCO will now be describedin connection with, e.g., a negative conductance LC oscillator circuit.

A conventional VCO is formed of an LC resonant circuit formed of twoinductor elements and two diode elements, and a positive feedbackcircuit formed of two transistors each having a gate connected to adrain of the other.

In this structure, an input impedance R_(in) of the positive feedbackcircuit is equal to −2/g_(m) (R_(in=−)2/g_(m)) where g_(m) represents amutual conductance of each transistor. Therefore, the VCO oscillateswhen an absolute value |R_(in)| of the input impedance is equal to orlower than an equivalent parallel resistance of the resonant circuit.Assuming that inductances L1 and L2 of the two inductor elements areboth equal to L (i.e., L1=L2=L) and a variable junction capacitance isequal to C_(var), an oscillation frequency f_(osc) is expressed by thefollowing formula (1): $\begin{matrix}{f_{osc} = \frac{1}{2\pi\sqrt{{LC}_{var}}}} & (1)\end{matrix}$

Accordingly, oscillation frequency f_(osc) can be controlled inaccordance with junction capacitance C_(var) varied by the controlvoltage connected to the diode element.

An oscillation amplitude A_(osc) of the VCO is expressed by thefollowing formula (2), and takes the value proportional to oscillationfrequency f_(osc).A_(osc)∝2πf_(osc)L   (2)

When the LC resonant circuit included in the VCO having the abovedifferential structure is to be used for 1 to 2 GHz, an LC type using alumped constant is predominantly employed because it can reduce an areaof an integrated structure. A variable capacitance (varactor diode) ispredominantly used as the capacity element. The inductor element isformed of a spiral inductance, which is formed of a spiralinterconnection and a leader interconnection, and is generally formed onthe same substrate as the transistor elements.

Accordingly, the inductance of the inductor element is uniquelydetermined in accordance with the form of the spiral, and cannot beadjusted unless a mask design is changed.

Meanwhile, the transistor elements formed on the same substrate do notnecessarily exhibit designed characteristics due to variations inmanufacturing steps. Therefore, inductance mismatching occurs betweenthe inductor elements, which reduces yield.

Recently, many kinds of inductance-variable elements, of whichinductance can be varied even after the inductor elements are assembledin circuits, have been proposed, e.g., in Japanese Patent Laying-OpenNos. 7-142258 and 8-162331.

For example, the inductance-variable element disclosed in JapanesePatent Laying-Open No. 7-142258 includes a spiral electrode formed on asemiconductor substrate with an insulating film therebetween and switchcircuits for short-circuiting various turn portions of the spiralelectrode.

In this structure, when the switch circuit is turned on in response to apredetermined applied voltage, the corresponding turn portion of thespiral electrode is locally short-circuited. This changes the number ofturns of the inductance-variable element so that the inductance-variableelement changes its inductance as a whole.

As already described, oscillation frequency f_(osc) in the conventionalVCO is controlled by variable capacitance C_(var). However, theequivalent parallel resistance of the LC resonant circuit lowers withincrease in variable capacitance C_(var). Therefore, VCO may deviatefrom an oscillation state if the capacitance value is high. Accordingly,it is difficult to achieve a wide oscillation frequency range.

Further, oscillation amplitude A_(osc) of the VCO is proportional tooscillation frequency f_(osc). In a low frequency range, therefore,oscillation amplitude A_(osc) is low, and a signal-to-noise ratio of theoscillation signal is low so that the phase noise characteristics areimpaired.

The foregoing inductance-variable element suffers from such a problemthat the Q value lowers due to an on-resistance of a switch circuitconnected in series to the inductor element. This results indeterioration of the phase noise characteristics of the oscillatorcircuit formed of the inductor element.

SUMMARY OF THE INVENTION

An object of the invention is to provide an oscillator circuit having awide oscillation frequency range and characteristics achieving low phasenoises.

Another object of the invention is to provide an L load differentialcircuit, which is mounted on the oscillator circuit, and achieves theabove performance.

According to an aspect of the invention, an oscillator circuit performsoscillation by positive feedback of an LC resonant circuit, and the LCresonant circuit includes a parallel resonant circuit that is formed ofan inductance-variable portion allowing variation of an inductance by aswitch circuit and a capacitor element.

According to another aspect of the invention, an oscillator circuit isformed of a pair of transistors cross-coupled to each other, and an LCresonant circuit of a differential type coupled to the pair oftransistors in a feedback manner. The LC resonant circuit includes firstand second inductance-variable portions including first and secondinput/output terminals, commonly connected at their second terminals toa fixed node and being capable of varying inductances, and a firstswitch circuit coupled between the first input/output terminals of thefirst and second inductance-variable portions. Each of the first andsecond inductance-variable portions has a spiral interconnection layerstarting from the first input/output terminal and formed on asemiconductor substrate with an interlayer insulating film therebetween,and a plurality of second switch circuits having first terminalsconnected to arbitrary positions on the interconnection layer and secondterminals commonly connected to the second input/output terminal,respectively. When one of the second switch circuits is turned on, theoscillator circuit electrically couples the connection position of theturned-on second switch circuit on the interconnection layer to thesecond input/output terminal. When the first switch circuit is turned onin response to the turn-on of the second switch circuit, the firstswitch circuit electrically couples the first and secondinductance-variable portions.

According to the invention described above, since the oscillationfrequency of the oscillator circuit is controlled by varying theinductance of the LC resonant circuit, it is possible to achieve theoscillator circuit, which can prevent deterioration of the phase noisecharacteristics in a low oscillation frequency range, and can achieve awide oscillation frequency range and characteristics ensuring low phasenoises.

Further, the two inductance-variable portions included in thedifferential LC resonant circuit are electrically coupled to form aninductor pair by the switch circuit arranged between theinductance-variable portions. Thereby, it is possible to suppressdeterioration of a Q value of the resonant circuit, and the voltagecontrol oscillator circuit can have characteristics ensuring low phasenoises. If the differential LC resonant circuit is configured not toconnect a capacitor element thereto, it can be used as an L loaddifferential circuit having a high Q value and a variable inductance.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows by way of example an oscillator circuit according to afirst embodiment of the invention.

FIG. 2 schematically shows by way of example a structure of aninductance-variable portion.

FIG. 3 shows by way of example a structure of a switch circuit.

FIG. 4 is an equivalent circuit diagram of the inductance-variableportion in FIG. 2.

FIG. 5 shows a circuit structure of the voltage control oscillatorcircuit in FIG. 1 having inductance-variable portions Lvar1 and Lvar2each formed of the inductance-variable portion shown in FIGS. 2 to 4.

FIG. 6 schematically shows a structure of a first modification of theinductance-variable portion in FIGS. 2 and 4.

FIGS. 7 to 9 are circuit diagrams showing structures of second, thirdand fourth modifications of the inductance-variable portion shown inFIGS. 2 and 4, respectively.

FIG. 10 is a circuit diagram showing by way of example a structure of anoscillator circuit according to a second embodiment of the invention.

FIG. 11 is a circuit diagram showing by way of example a structure of anoscillator circuit according to a third embodiment of the invention.

FIG. 12 is a circuit diagram schematically showing a structure of aswitch circuit group 1 in a voltage control oscillator circuit in FIG.11.

FIG. 13 is an equivalent circuit diagram of switch circuit group 1 inFIG. 12.

FIG. 14 is an equivalent circuit diagram of switch circuit group 1changed from Δ-connection to Y-connection shown in FIG. 13.

FIG. 15 shows a specific layout structure of inductance-variableportions Lvar1 and Lvar2 shown in FIG. 11.

FIG. 16 is a circuit diagram showing by way of example a structure of anoscillator circuit according to a modification of the third embodimentof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention will now be described with reference to thedrawings. In the figures, the same reference numbers indicate the sameor corresponding portions.

First Embodiment

FIG. 1 shows a structure of an oscillator circuit according to a firstembodiment of the invention. In the following description of theembodiment, a voltage control oscillator circuit will be described as anexample of the oscillator circuit.

Referring to FIG. 1, a voltage control oscillator circuit is formed of adifferential type LC resonant circuit, which is formed ofinductance-variable portions Lvar1 and Lvar2 having variable inductancesand a capacitor element C1, and a positive feedback circuit formed ofN-channel MOS transistors M1 and M2.

Each of inductance-variable portions Lvar1 and Lvar2 has first andsecond input/output terminals, and the second input/output terminal iscommonly connected to an external power supply node Vdd. The firstinput/output terminals are connected to output nodes OUT and OUTB,respectively. Capacitor element C1 is connected between the firstinput/output terminals of inductance-variable portions Lvar1 and Lvar2.An oscillation frequency f_(osc) of the voltage control oscillatorcircuit can be determined based on the inductance values of theinductance-variable portions and the capacitance value.

The positive feedback circuit includes N-channel MOS transistor M1electrically coupled between inductance-variable portion Lvar1 and aconstant current supply Ibias, and N-channel MOS transistor M2electrically coupled between inductance-variable portion Lvar2 andconstant current supply Ibias.

N-channel MOS transistors M1 and M2 have gates each coupled to a drainof the other, and thus provide a cross-coupled structure.

An operation of the voltage control oscillator circuit shown in FIG. 1will now be described.

Referring to FIG. 1, since the positive feedback circuit of the voltagecontrol oscillator circuit can be deemed as a two-terminal circuit, aninput impedance R_(in) viewed from the drains of N-channel MOStransistors M1 and M2 can be expressed as R_(in)=−2/g_(m), where g_(m)is a mutual conductance of each N-channel MOS transistor. Therefore,this circuit oscillates when an absolute value |R_(in)| of inputimpedance R_(in) is equal to or lower than a value of an equivalentparallel resistance of the LC resonant circuit. This circuit may bereferred to as a “negative conductance LC oscillator circuit”.

When the circuit satisfies the foregoing oscillation conditions,oscillation frequency f_(osc) is expressed by the following formula (3),where L indicates an inductance value of inductance-variable portionsLvar1 and Lvar2, and C₁ indicates a capacitance value of capacitorelement C1. Parasitic capacitances of the respective passive elements,interconnections and others are ignored. $\begin{matrix}{f_{osc} = \frac{1}{2\pi\sqrt{{LC}_{1}}}} & (3)\end{matrix}$

An oscillation amplitude Aosc is expressed by the following formula (4):A_(osc)∝2πf_(osc)L   (4)

As can be seen from the formula (3), oscillation frequency f_(osc)changes in accordance with inductance value L. For example, oscillationfrequency f_(osc) lowers with increase in inductance value L. In thiscase, since oscillation frequency f_(osc) is lowered in accordance withincrease in inductance value L, deterioration of oscillation amplitudeA_(osc) expressed in FIG. (4) is prevented. Therefore, it is possible toavoid deterioration of the phase noise characteristics, which occurs dueto lowering of the oscillation amplitude in a conventional VCO when theoscillation frequency is in a low range.

Then, description will be given on a specific example of the structuresof inductance-variable portions Lvar1 and Lvar2 forming the LC resonantcircuit in the voltage control oscillator circuit shown in FIG. 1.

FIG. 2 schematically shows by of example a structure ofinductance-variable portions Lvar1 and Lvar2. Since inductance-variableportions Lvar1 and Lvar2 have the same structure, FIG. 2representatively shows only inductance-variable portion Lvar1.

Referring to FIG. 2, inductance-variable portion Lvar1 includes a spiralinterconnection layer formed on a semiconductor substrate (not shown)with an interlayer insulating film therebetween, and switch circuitsSW1-SW3.

The spiral interconnection layer is made of a metal material such asaluminum or copper, and the configuration thereof is not restricted to asquare, and may be another form such as a polygon or a circle.

Switch circuits SW1-SW3 have first terminals, which are connected torespective turns of the spiral interconnection layer, and secondterminals connected to an input/output terminal of the inductor element.Switch circuits SW1-SW3 receive control signals for controlling theturning-on/off thereof.

FIG. 3 shows by way of example a structure of switch circuits SW1-SW3.

Referring to FIG. 3, switch circuit SWn (n=1, 2 or 3) may be formed ofan N-channel MOS transistor 10. When N-channel MOS transistor 10 issupplied with a control voltage Vsw as a control signal Sn on its gate,it is turned on or off depending on the voltage level of control voltageVsw. When control voltage Vsw is at a H-level (high potential level),N-channel MOS transistor 10 is turned on so that the correspondingportion of the spiral interconnection layer is electrically coupled tothe input/output terminal of the inductor element. When control voltageVsw is at a L-level (low potential level), N-channel MOS transistor 10is turned off. Thereby, the corresponding portion of the spiralinterconnection layer is electrically isolated from the input/outputterminal of the inductor element.

Accordingly, one of the switch circuits is selected to receive controlvoltage Vsw at the H-level, and the other switch circuits are suppliedwith control voltage Vsw at L-level so that an intended inductance valuecan be obtained.

In the inductance-variable portion of the structure shown in FIG. 2,since switch circuits SW1-SW3 are provided for the respective turns ofthe spiral interconnection layer, discrete inductance values can beobtained.

In FIG. 3, the N-channel MOS transistor is used as the switch circuit.However, a bipolar transistor or a GaAs MESFET (Metal SemiconductorField-Effect Transistor) may be used instead of the N-channel MOStransistor.

FIG. 4 is an equivalent circuit diagram of inductance-variable portionLvar1 in FIG. 2.

Referring to FIG. 4, the inductance-variable portion is divided intothree inductor elements L1, L2 and L3 by switch circuits SW1-SW3arranged for the respective turns. It is assumed that inductor elementsL1, L2 and L3 have inductance values of L1, L2 and L3, respectively.

For example, when switch circuit SW1 is on, the whole inductor elementshave the inductance value of L1. When switch circuit SW2 is on, thewhole inductor elements have the inductance value of (L1+L2). In thismanner, one of switch circuits SW1-SW3 is turned on so that theinductance can selectively take the discrete values within a variablerange from L1 to (L1+L2+L3).

FIG. 5 shows a circuit structure, in which each of inductance-variableportions Lvar1 and Lvar2 in the voltage control oscillator circuit shownin FIG. 1 employs the inductance-variable portion shown in FIGS. 2 to 4.

In the voltage control oscillator circuit in FIG. 5, inductance-variableportions Lvar1 and Lvar2 in the LC resonant circuit shown in FIG. 1 areexpressed as the equivalent circuits shown in FIG. 4, and switchcircuits SW1-SW3 and SW1 d-SW3 d are arranged for the respective turns.Capacitor element C1 in the LC resonant circuit and the circuitstructure of the positive feedback circuit are similar to those of theVCO in FIG. 1, and therefore, description thereof is not repeated.

Switch circuits SW1 and SW1 d form one switch circuit group. Likewise,switch circuits SW2 and SW2 d form one switch circuit group, and switchcircuits SW3 and SW3 d form one switch circuit group.

In this structure, one switch circuit group is selected from the threeswitch circuit groups, and switch circuits SWn and SWnd in the selectedgroup are turned on. The switch circuits in the other switch circuitgroups are kept off. For example, when switch circuits SW1 and SW1 d areturned on, each of inductance-variable portions Lvar1 and Lvar2 takesthe inductance value of L1. Thus, by turning on one of the switchcircuit groups, the inductance of the inductance-variable portion can bediscretely varied within the variable range from L1 to (L1+L2+L3), asalready described. As a result, the variable range of oscillationfrequency f_(osc) of the voltage control oscillator circuit can beexpressed by the following formula (5): $\begin{matrix}{\frac{1}{2\pi\sqrt{\left( {L_{1} + L_{2} + L_{3}} \right)C}} \leqq f_{osc} \leqq \frac{1}{2\pi\sqrt{L_{1}C}}} & (5)\end{matrix}$

Even in the low frequency range within the variable oscillationfrequency range, oscillation amplitude A_(osc) does not deteriorateowing to increase in inductance L so that deterioration of phase noisesdoes not occur.

Therefore, the first embodiment of the invention can achieve the voltagecontrol oscillator circuit having a wide oscillation frequency range andlow-phase-noise characteristics, i.e., characteristics ensuring lowphase noises.

Modification of First Embodiment

As described above, the oscillator circuit of the first embodimentincludes the LC resonant circuit employing the inductance-variableportion for improving the trade-off relationship between the variableoscillation frequency range and the phase noise characteristics. Theinductance-variable portion can easily provide various inductance valuesby switching the plurality of switch circuits provided for the spiralinterconnection layer of the inductor element. A modification of thestructure of the inductance-variable portion will now be described.

FIG. 6 schematically shows a structure of a first modification ofinductance-variable portion Lvar1 shown in FIGS. 2 and 4.Inductance-variable portion Lvar2 has the same structure asinductance-variable portion Lvar1, and therefore description thereof isnot repeated.

Referring to FIG. 6, inductance-variable portion Lvar1 includes switchcircuits SW1-SW4 arranged for quarters of the turn of the spiralinterconnection layer, respectively, and thus has a structure achievedby adding a switch circuit to the inductor element in FIGS. 2 and 4.

In this structure, an intended inductance can be likewise achieved byturning on one of switch circuits SW1-SW4. Further, by increasing thenumber of the switch circuits, it is possible to widen the variablerange of the inductance value and to perform the control more finely.

Therefore, by mounting the inductance varying portion in FIG. 6 on theLC resonant circuit of the voltage control oscillator circuit shown inFIG. 1, it is possible to widen the variable range of oscillatorfrequency f_(osc) and to perform the control more finely. The number ofthe switch circuits and the positions of connection to the spiralinterconnection layer are not restricted to those of this embodiment,and can be arbitrarily changed so that an intended oscillation frequencycan be achieved.

Further, lowering of oscillation amplitude A_(osc) can be suppressedowing to a large inductance even in the low frequency range within thevariable frequency range, and therefore deterioration of the phase noisecharacteristics can be avoided.

Second Modification of the First Embodiment

FIG. 7 is an equivalent circuit diagram showing a structure of a secondmodification of inductance-variable portion Lvar1 shown in FIGS. 2 and4.

Referring to FIG. 7, inductance-variable portion Lvar1 includes switchcircuits SW4 and SW5 in addition to switch circuits SW1-SW3 arranged forrespective turns in the equivalent circuit of the inductance-variableportion shown in FIG. 4.

Switch circuit SW4 is arranged between input/output terminals 1 and 2,and is connected in parallel with inductor elements L1-L3. Switchcircuit SW5 is arranged between input/output terminal 1 and the terminalof switch circuit SW2, and is connected in parallel with inductorelements L1 and L2.

In this structure, switch circuits SW1-SW5 are selectively turned on sothat the inductance can be varied more finely in stepwise fashion. Forexample, when only switch circuit SW1 is turned on, the inductance valueof L1 is achieved. When only switch circuit SW2 is turned on, theinductance value is equal to (L1+L2). Likewise, when switch circuit SW3is turned on, the inductance value is equal to (L1+L2+L3).

When switch circuits SW4 and SW5 are turned on, the inductance value issubstantially equal to zero. When switch circuits SW5 and SW3 are turnedon, the inductance value is equal to L3.

As described above, the inductance can be finely varied by variouslycombining the on and off states of the plurality of switch circuits.Therefore, by employing the inductance-variable portion in FIG. 7 in theLC resonant circuit of the voltage control oscillator circuit shown inFIG. 1, it is possible to widen the variable frequency range ofoscillation frequency f_(osc) and to perform the control more finely.

Third Modification of the First Embodiment

FIG. 8 shows a structure of a third modification of inductance-variableportion Lvar1 shown in FIGS. 2 and 4.

Referring to FIG. 8, inductance-variable portion Lvar1 includes switchcircuits SW4-SW9 in addition to switch circuits SW1-SW3 provided for therespective turns in the equivalent circuit of the inductor element shownin FIG. 2.

Switch circuits SW4-SW6 are connected in parallel with inductor elementsL1-L3, respectively. Switch circuit SW7 is connected between one end ofinductor element L2 and one end of inductor element L3, and is arrangedin parallel with inductor elements L2 and L3. Switch circuit SW8 isconnected between one end of inductor element L1 and one end of inductorelement L2, and is arranged in parallel with inductor elements L1 andL2. Switch circuit SW9 is connected between one end of inductor elementL1 and one end of inductor element L3, and is arranged in parallel withinductor elements L1, L2 and L3.

In this structure, switch circuits SW1-SW9 are selectively turned on sothat the inductance can be controlled more finely that theinductance-variable portion shown in FIGS. 2 and 7.

For example, when switch circuits SW2 and SW4 are turned on, theinductance value of L2 is obtained. When switch circuits SW3 and SW8 areturned on, the inductance value of L3 is obtained. When switch circuitsSW3 and SW4 are turned on, the inductance value of (L2+L3) is obtained.

As described above, the inductance can be determined more finely in thevariation range by combining the on/off states of the plurality ofswitch circuits. Therefore, by employing the inductance-variable portionshown in FIG. 8 in the LC resonant circuit of the voltage controloscillator circuit in FIG. 1, it is possible to widen the variablefrequency range of oscillation frequency f_(osc), and the control can beperformed more finely.

Fourth Modification of the First Embodiment

FIG. 9 is a circuit diagram showing a structure of a fourth modificationof inductance-variable portion Lvar1 shown in FIG. 2.

Referring to FIG. 9, inductance-variable portion Lvar1 includes aplurality of inductor elements L1-L3 having different inductances,respectively, and switch circuits SW1-SW3 each coupled between one endof the spiral interconnection layer (not shown) of correspondinginductor element L1, L2 or L3 and the input/output terminal.

The inductance-variable portion in FIG. 2 has the plurality of switchcircuits arranged for the one spiral interconnection layer. In contrastto this, the inductor element shown in FIG. 9 includes the switchcircuits provided for the respective spiral interconnection layers in aone-to-one relationship. In the inductance-variable portion shown inFIG. 9, therefore, the inductance can be varied by turning on only theswitch circuit, which corresponds to the inductor element having anintended inductance.

According to the inductance-variable portion having the above structure,the plurality of spiral interconnection layers are arranged in parallel,and therefore the circuit scale is large. However, the switch circuitper one inductor element is small in number so that the circuitstructure can be simple.

Second Embodiment

FIG. 10 shows by way of example an oscillator circuit according to asecond embodiment of the invention. Similarly to the first embodiment, avoltage control oscillator circuit will be described as an example ofthe oscillator circuit.

Referring to FIG. 10, the voltage control oscillator circuit differsfrom the voltage control oscillator circuit shown in FIG. 1 only in thatthe capacitor element forming the LC resonant circuit has a variablecapacitance. Description of the same or corresponding portions is notrepeated.

The LC resonant circuit is formed of inductance-variable portions Lvar1and Lvar2 each connected between external power supply node Vdd andoutput node OUT or OUTB, and a variable capacitor element Cvar connectedbetween first input/output terminals of inductor elements Lvar1 andLvar2. In the following description, it is assumed that each passiveelement has an inductance of L and a capacitance value of C.

In this structure, oscillation frequency f_(osc) of the voltage controloscillator circuit is expressed by the following formula (6), in whichparasitic capacitances and others of each passive element,interconnection and others are ignored. $\begin{matrix}{f_{osc} = \frac{1}{2\pi\sqrt{LC}}} & (6)\end{matrix}$

Oscillation amplitude A_(osc) is expressed by the following formula (7).A_(osc)∝2πf_(osc)·L   (7)

As can be seen from the formula (6), oscillation frequency f_(osc)depends on a combination of two variables, i.e., inductance L andcapacitance value C. Therefore, the variable range of the oscillationfrequency can be wider than that in the voltage control oscillatorcircuit of the first embodiment, in which only the inductance isvariable.

Since the oscillation frequency can be lowered by increasing inductanceL similarly to the first embodiment, deterioration of oscillationamplitude A_(osc) can be suppressed even in the low oscillationfrequency range. Thereby, deterioration of the phase noisecharacteristics at the low oscillation frequencies can be suppressed sothat the trade-off relationship between the variable range of theoscillation frequency and the phase noises can be improved.

Third Embodiment

FIG. 11 shows a structure of an oscillator circuit according to a thirdembodiment of the invention. A voltage control oscillator circuit willnow be described as an example of an oscillator circuit.

Referring to FIG. 11, the voltage control oscillator circuit includesswitch circuits SW1 dd-SW3 dd arranged between inductance-variableportions Lvar1 and Lvar2 of the differential LC resonant circuit, inaddition to the components of the voltage control oscillator circuit ofthe first embodiment shown in FIG. 5. Description of the same orcorresponding portions is not repeated.

Inductance-variable portions Lvar1 and Lvar2 include switch circuitsSW1-SW3 or SW1 d-SW3 d arranged corresponding to the respective turns,similarly to inductance-variable portion Lvar1 shown in FIG. 2.

Further, a switch circuit SW1 dd is arranged between switch circuits SW1and SW1 d. A switch circuit SW2 dd is arranged between switch circuitsSW2 and SW2 d. A switch circuit SW3 dd is arranged between switchcircuits SW3 and SW3 d. Switch circuits SW1, SW1 d and SW1 dd form oneswitch circuit group 1, switch circuits SW2, SW2 d and SW2 dd form aswitch circuit group 2, and switch circuits SW3, SW3 d and SW3 dd formone switch circuit group 3.

By selecting one of switch circuit groups 1-3, switch circuits SWn, SWndand SWndd (n=1, 2 or 3) forming the selected switch circuit group areall turned on. Consequently, inductance-variable portions Lvar1 andLvar2 are electrically coupled to form an inductor pair.

FIG. 12 schematically shows a structure of switch circuit group 1, 2 or3 in the voltage control oscillator circuit shown in FIG. 11. Since theswitch circuit groups 1-3 have the same structure, the structure ofswitch circuit group 1 will now be representatively described.

As shown in FIG. 12, switch circuits SW1 and SW1 d are connected inparallel between external power supply node Vdd and inductor element L1.Further, switch circuit SW1 dd is coupled between switch circuits SW1and SW1 d.

Effects by switch circuits SW1 dd-SW3 dd are as follows.

In the voltage control oscillator circuit shown in FIG. 11, one of theswitch circuit groups is selected and turned on. For example, whenswitch circuit group 1 is selected, switch circuits SW1, SW1 d and SW1dd are turned on. Thereby, the inductance equal to L1 is set in each ofinductance-variable portions Lvar1 and Lvar2 arranged between externalpower supply node Vdd and respective output nodes OUT and OUTB.

Further, inductance-variable portions Lvar1 and Lvar2 are in the state,where these portions are electrically coupled via switch circuit SW1 dd.An equivalent circuit of only switch circuit group 1 in this state isshown in FIG. 13. A resistance element R is an on-resistance of eachswitch circuit.

In the equivalent circuit formed of three resistance elements R shown inFIG. 13, Δ-connection of resistance elements R may be changed intoY-connection, whereby switch circuit group 1 changes into an equivalentcircuit shown in FIG. 14. As shown in FIG. 14, each of the threeresistance elements forming the equivalent circuit has a resistancevalue of R/3. Therefore, a resistance component, which is connected inseries to each of the inductor elements included in inductance-variableportions Lvar1 and Lvar2 in FIG. 11, has a resistance value of R3.

Meanwhile, in each of the inductance-variable portions of the voltagecontrol oscillator circuit shown in FIG. 5, a resistance componentconnected in series to the inductor element has the resistance value ofR equal to the on-resistance of switch circuits SW1-SW3 and SW1 d-SW3 d.Thus, interposition of switch circuits SW1 dd-SW3 dd reduces theresistance values of resistance components to ⅓.

The Q value of the LC resonant circuit has such characteristics that theQ value of the LC resonant circuit rises with decrease in resistancecomponent connected in series to the inductor element, and lowers withincrease in resistance component. Therefore, the resonant circuit in thevoltage control oscillator circuit of this embodiment can have a higherQ value than the LC resonant circuit in FIG. 5 owing to the reduction inresistance component by switch circuits SW1 dd-SW3 dd. This results inlow-phase-noise characteristics of the voltage control oscillator.

The differential LC resonant circuit thus constructed can be applied notonly to the voltage control oscillator circuit of this embodiment, butalso can be applied to an RF circuit such as a differential amplifierand a mixer, which has a differential LC resonant circuit as a load forachieving high-gain characteristics and low-noise characteristics owingto a high Q value. Without connecting a capacitor element, the circuitmay be used merely as an L load differential circuit in the RF circuitor the like, in which case a circuit having a variable gain can beachieved owing to the feature that the inductance value is variable.

FIG. 15 shows a specific layout structure of inductance-variableportions Lvar1 and Lvar2 in the differential LC resonant circuitincluded in the voltage control oscillator circuit shown in FIG. 11.

Referring to FIG. 15, inductance-variable portions Lvar1 and Lvar2 forma differential inductance including a combination of two spiralinterconnection layers. Input/output terminal 1 common to the twoinductance-variable portions is connected to external power supply nodeVdd (not shown in FIG. 15). Other input/output terminals 2 and 3 of theinductance-variable portions are connected to output nodes OUT and OUTBof the voltage control oscillator circuit (not shown in FIG. 15),respectively.

By providing the two inductance-variable portions formed of thedifferential type inductors as shown in FIG. 15, switch circuits SW1dd-SW3 dd can be interposed without increasing the circuit scale, andthus the structure can be compact.

According to the third embodiment of the invention, as described above,the two inductance-variable portions are electrically coupled by theswitch circuits arranged therebetween to form the one inductor pair, andthereby the resistance component connected in series to the inductorelement can be reduced so that deterioration of the Q value of thedifferential LC resonant circuit can be suppressed, and the voltagecontrol oscillator circuit can have the characteristics ensuring lowphase noises.

In the differential LC resonant circuit, the inductor pair is formed ofthe differential type inductors. Thereby, it is possible to suppressincrease in circuit scale, which may be caused by interposition of theswitch circuits, and the voltage control oscillator circuit can becompact in layout.

Modification of the Third Embodiment

FIG. 16 is a circuit diagram showing a structure of a voltage controloscillator circuit, which is an oscillator circuit according to amodification of the third embodiment of the invention.

Referring to FIG. 16, the voltage control oscillator circuit differsfrom the voltage control oscillator circuit shown in FIG. 11 in that theinductor pair included in the differential LC resonant circuit is formedof inductance-variable portions Lvar1 and Lvar2, each of which is formedof a plurality of inductor elements, and switch circuits SW1 dd-SW3 dd.Therefore, description of the portions corresponding to those of thevoltage control oscillator circuit in FIG. 11 is not repeated.

The inductor pair is formed of two inductance-variable portions Lvar1and Lvar2, which are arranged in parallel and are connected to externalpower supply node Vdd, and switch circuits SW1 dd-SW3 dd arrangedbetween inductance-variable portions Lvar1 and Lvar2.

Inductance-variable portions Lvar1 and Lvar2 have the same structures asthose shown in FIG. 9. Inductance-variable portion Lvar1 includes aplurality of inductor elements L1-L3, which are connected in parallelbetween external power supply node Vdd and output node OUT of thevoltage control oscillator circuit, and have different inductances,respectively. Also, inductance-variable portion Lvar1 includes switchcircuits SW1-SW3 coupled between respective inductor elements L1-L3 andexternal power supply node Vdd. Likewise, inductance-variable portionLvar2 includes a plurality of inductor elements L1-L3, which areconnected in parallel between external power supply node Vdd and outputnode OUTB of the voltage control oscillator circuit, and have differentinductances, respectively, and switch circuits SW1 d-SW3 d coupledbetween respective inductor elements L1-L3 and external power supplynode Vdd.

In this structure, an intended inductance can be achieved in each ofinductance-variable portions Lvar1 and Lvar2 by turning on one of theplurality of switch circuits SW1-SW3 or SW1 d-SW3 d.

At the same time as the turn-on of switch circuits SWn and SWnd,corresponding one of switch circuits SW1 dd-SW3 dd arranged between theinductance-variable portions is turned on to form the inductor pair.Therefore, a resistance component, which is connected in series to eachof the inductor elements, is reduced to R/3, as is done in the thirdembodiment. Thereby, a high Q value is achieved in the differential LCresonant circuit so that the voltage control oscillator circuit can havethe characteristics ensuring the low-phase noises.

Similarly to the third embodiment, the differential LC resonant circuitthus constructed can also be applied to an RF circuit such as adifferential amplifier and a mixer, and thereby high-gaincharacteristics and low-noise characteristics can be achieved owing tothe high Q value. Without connecting the capacitor element, the circuitmay be used merely as an L load differential circuit in the RF circuitor the like, in which case a circuit having a variable gain can beachieved owing to the feature that the inductance value is variable.

As already described in connection with the first to third embodiments,the oscillator circuit according to the invention can improve thetrade-off relationship between the variable frequency range and thephase noise characteristics owing to the LC resonant circuit, which isconfigured to perform the control by the switch circuits arrangedcorresponding to the portions of the spiral interconnection layer, andthereby to provide the variable inductance values for controlling theoscillation frequency.

Further, according to the third embodiment, the differential LC resonantcircuit includes the two inductance-variable portions, which areelectrically coupled via the switch circuits to provide the inductorpair. Thereby, the resistance element connected in series to theinductor element is reduced, and the high Q value can be achieved. Byarranging the resonant circuit thus constructed in the voltage controloscillator circuit, low-phase-noise characteristics can be achieved.

Since the insertion loss caused by the switch circuits still exerts alarge influence on the Q value of the resonant circuit and the phasenoise characteristics of the voltage control oscillator circuit, it isdesired to lower further the insertion loss.

Accordingly, the switch circuit may be formed of a transistor such as aDepletion-layer-Extended Transistor (which may be referred to as a“DTE”, hereinafter) capable of reducing an insertion loss, whereby thephase noise characteristics can be further improved.

The DTE has an element structure, which can be formed by removing aP-type well, P⁺-isolation layer and a punch-through stopper layer from aconventional CMOS transistor, and achieves a low junction capacitance ofsource/drain electrodes and a high ground resistance so that a lowinsertion loss can be achieved. Specific element structures of the DTEare disclosed, e.g., in “A 1.4 dB Insertion-Loss, 5 GHz Transmit/ReceiveSwitch Utilizing Novel Depletion-Layer-Extended Transistors (DETs) in0.18 μm CMOS Process”, T. Ohnakado, et al., IEEE Symposium on VLSITechnology Digest of Tech. Papers, 16.4, June 2002.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. An oscillator circuit, comprising: a pair of transistorscross-coupled to each other; and an LC resonant circuit of adifferential type coupled to said pair of transistors in a feedbackmanner; wherein said LC resonant circuit includes first and secondinductance-variable portions including first and second input/outputterminals, said second input/output terminals being commonly connectedto a fixed node, and said first and second inductance-variable portionsbeing capable of varying inductances, and a first switch circuit coupledbetween the first input/output terminals of said first and secondinductance-variable portions, each of said first and secondinductance-variable portions has a plurality of spiral interconnectionlayers starting from said first input/output terminal, and a pluralityof second switch circuits coupled between trailing ends of saidplurality of interconnection layers and said second input/outputterminal, respectively, when one of said plurality of second switchcircuits is turned on, the trailing end of said interconnection layerincluded in said plurality of interconnection layers and connected tosaid turned-on second switch circuit is electrically coupled to saidsecond input/output terminal, and when said first switch circuit isturned on in response to the turn-on of said second switch circuit, saidfirst switch circuit electrically couples said first and secondinductance-variable portions.